Expansion is a very critical process that affects the final structural, mechanical and textural characteristics of the extruded products. An understanding of the expansion phenomenon is necessary in order to control and/or improve the functionality of starch products. The hybrid mixture theory based model developed by Takhar (2014) was used to obtain a computer model, which can be used to design, optimize and/or control the process conditions, and improve the specific characteristics of expanded biopolymers. Water, vapor and heat transport mechanisms and thermo–mechanical changes occurring inside the expanding extrudate were described using the two–scale unsaturated transport equations coupled with the poroviscoelasticity equations. The poroviscoelastic extrudate matrix interacted with the fluids (liquid water, vapor and air mixture) contained in its pores. Transport equations were transformed from the moving Eulerian coordinates to the stationary Lagrangian coordinates. Good agreements between the simulated and the experimental values of surface temperature, moisture content and expansion ratio of the extrudates at different extrusion conditions were obtained. Simulations showed the transition from rubbery to glassy state at different spatial locations across the cross–section of the extrudate. The model was also used to predict the temperature, moisture content, and pressure distribution within the extrudate. The extrudate moisture content deceased after exiting from the die due to water evaporation, which resulted in an increase in the glass transition temperature. Therefore, a glassy crust was developed from the surface to interior. An increase in pore pressure of the extrudate caused rapid starch expansion until it reached a maximum. The extrudate stopped expanding when the pore pressure gradient became zero, and collapsed when the pore pressure reduced further to make a negative pressure gradient.
The viscoelastic properties also provide an indication about the textural characteristics of starch as a function of extrusion conditions. The viscoelastic properties of extruded cornstarch were measured as a function of process conditions. The extruded cornstarch samples were conditioned in the moisture range of 12.9–31.2% (dry basis). Dynamic and creep behavior of the extrudate were evaluated using a dynamic mechanical analyzer. The storage (G′) and loss moduli (G′′) were determined in the temperature range of 30-100°C and oscillation frequency range of 0.1-51 Hz. The G′ and G″ values showed an increasing trend with a decrease in temperature and moisture content and an increase in frequency. This implied an increase in solid-like behavior when the temperature and moisture content were reduced or the frequency was increased. Rheological parameters based on creep behavior were assessed at 30, 60 and 85°C. Creep data, fitted using Burger’s model, were highly dependent on both moisture content and temperature. The viscoelastic properties obtained in this experimental research were used in the transport model for studying the expansion of starch during extrusion.
For validating the predictive model, expansion experiments were conducted. Three response parameters (apparent density, porosity and expansion ratio) were examined for investigating the expansion characteristics of cornstarch as a function of extrusion conditions. Cornstarch with moisture contents of 18.3, 26.1 and 34.8% (dry basis) were extrusion cooked in a twin-screw extruder at barrel temperatures of 120 and 140°C and screw speeds of 200, 300 and 400 rpm. Results indicated that feed moisture content, barrel temperature and screw speed had significant effects on the density, porosity and expansion ratio of extruded cornstarch products (p<0.05). Increases in feed moisture content resulted in increases in bulk density and decreases in porosity and expansion ratio (p<0.05). Increases in barrel temperature and screw speed resulted in decreases in bulk density and increase in porosity and expansion ratio (p<0.05). In addition, the moisture content and surface temperature of the extruded products were influenced by extrusion conditions. Lowering the feed moisture content and increasing the barrel temperature and screw speed caused a lower moisture content and higher surface temperature of the cornstarch extrudates (p<0.05).